The nords are a "warrior" culture. Everything's okay so long as you're bigger, swing a bigger axe, and make sure everyone else knows it. It's all about the one.
The Imperials are essentially a collectivist culture. An organized many are more powerful than any one, no matter how great they think they are. There is no I, there is only we, and we are all that matter. We will endure. We will survive. We will prevail. Individuals may rise to prominence, influence great events, amass great wealth, do great deeds, but every one of them is a piece of the greater machine.
Typical nords see this as weakness, and love to pile on abuse, insults, slander and mockery. It's a defensive mechanism to overcompensate for them knowing deep down that their entire society is doomed to total failure in the future. It's rather amusing in a sophomoric, juvenile way.